Method to compensate for stress between heat spreader and thermal interface material

ABSTRACT

A device and method for identifying and compensating for tensile stress due to heat caused expansion and contraction between an integrated heat spreader and thermal interface material. This device and method will change the shape of the integrated heat spreader based upon the identification of the location of the highest tensile and/or shear stress so that additional thermal interface material is deposited between the integrated heat spreader and a die. Utilizing this method and device, heat is efficiently transferred from the die to the integrated heat spreader.

FIELD

[0001] The invention relates to a device and method to control strainand tensile stress on thermal interface material in a heat spreader.More particularly, the present invention is a device and method whichdetermines the stress points in thermal interface material used totransfer heat from a die to a heat spreader and designs the heatspreader to optimize the thickness of the thermal interface material forthose stress points.

BACKGROUND

[0002] In the rapid development of computers many advancements have beenseen in the areas of processor speed, throughput, communications, faulttolerance and size of individual components. Today's microprocessors,memory and other chips have become faster and smaller. However, with theincrease in speed, reduction in the size of components, and increaseddensity of circuitry found within a given chip/die, heat generation anddissipation have become a more critical factor than ever.

[0003]FIG. 1 illustrates a die 50 placed on a substrate 30 with a finiteamount of a thermal interface material (TIM) 20 placed on top of the die50. This TIM 20 serves at least two primary purposes. First, it acts toconduct heat from the die to the integrated heat spreader (IHS) 10.Second, it may also provide some adhesion between the IHS 10 and die 50.The TIM 20 may be composed of, but not limited to, solder, a polymercontaining metal, or some other substances which both acts to transferheat and provide some adhesion. During the manufacturing process the IHS10 is pressed down upon the TIM 20 and adhesive 40 resulting in astructure as shown in FIG. 2.

[0004] As shown in FIG. 2, the IHS 10 would absorb heat from die 50through TIM 20 and be held in place on the substrate 30 via adhesive 40.On top of the IHS 10 a heat sink (not shown) or fan/heat sinkcombination (not shown) would be mounted to dissipate the heat absorbedby the IHS 10. However, since IHS 10 and TIM 20 both experiencesignificant tensile stress during the assembly process and due tothermal expansion and contraction when the die is powered on and off, asshown in FIG. 3, air gaps 60 form between the TIM 20 and IHS 10. Asindicated in FIG. 3, the these air gaps 60 may form at the outer edgesof the TIM 20 while the center portion of the TIM 20 remains in contactwith the IHS 10.

[0005] However, as shown in FIGS. 3 and 4, the air gap 60 may occuranywhere in the contact area between TIM 20 and IHS 10. As illustratedin FIG. 4, the air gap 60 may form in the center of the contact areabetween the TIM 20 and IHS 10 while the outer edges of the TIM 20 remainin contact with the IHS 10.

[0006] As would be appreciated by one of ordinary skill in the art,these air gaps 60 shown in FIGS. 3 and 4 may form anywhere in thecontact area between the TIM 20 and IHS 10 depending on the materialsutilized in the IHS 10 and TIM 20 as well as the handling procedures forthe IHS 10 during the manufacturing process. Further, these air gaps 60may also form in the TIM 20 itself. It should be noted that FIGS. 3 and4, except for the inclusion of air gaps 60, remain unchanged from thatshown in FIG. 2 and will not be discussed in further detail.

[0007] Since separation may occur between the TIM 20 and IHS 10 formingair gaps 60, as shown in FIGS. 3 and 4, due to thermal expansion andcontraction, these air gaps 60 act as insulation preventing heat beingtransferred from the die 50 to the IHS 10. As heat builds up in the die50 to higher levels, the life expectancy of the die 50 is reduced.

[0008] Therefore, what is needed is a device and method that candetermine the stress points between the TIM 20 and IHS 10 due to thermalexpansion and contraction. Further, what is needed is a device andmethod that may compensate for the tensile and shear stress therebypreventing the separation of the TIM 20 and the IHS 10. Still further,what is needed is a device and method that will provide for efficientheat transfer from the die 50 to the IHS 10.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The foregoing and a better understanding of the present inventionwill become apparent from the following detailed description ofexemplary embodiments and the claims when read in connection with theaccompanying drawings, all forming a part of the disclosure of thisinvention. While the foregoing and following written and illustrateddisclosure focuses on disclosing example embodiments of the invention,it should be clearly understood that the same is by way of illustrationand example only and the invention is not limited thereto. The spiritand scope of the present invention are limited only by the terms of theappended claims.

[0010] The following represents brief descriptions of the drawings,wherein:

[0011]FIG. 1 is an example of an integrated heat spreader (IHS) beingaffixed to a die attached to a substrate;

[0012]FIG. 2 is an example of an assembled integrated heat spreader(IHS) and die with a thermal interface material (TIM) to conduct heatfrom the die to the integrated heat spreader (IHS);

[0013]FIG. 3 is an example of an assembled IHS and die with a TIM thathas separated from the IHS to form air gaps;

[0014]FIG. 4 is an example of an assembled IHS and die with a TIM thathas separated from the IHS to form an air gap;

[0015]FIG. 5 is an assembled convex IHS in an example embodiment of thepresent invention;

[0016]FIG. 6 is an assembled concave IHS in an example embodiment of thepresent invention; and

[0017]FIG. 7 is a flowchart of a process to determine the stress pointsin a TIM and modify the IHS to compensate for the stress points in anexample embodiment of the present invention.

DETAILED DESCRIPTION

[0018] Before beginning a detailed description of the subject invention,mention of the following is in order. When appropriate, like referencenumerals and characters may be used to designate identical,corresponding or similar components in differing figure drawings.Further, in the detailed description to follow, exemplarysizes/models/values/ranges may be given, although the present inventionis not limited to the same. As a final note, well-known components ofcomputer networks may not be shown within the FIGS. for simplicity ofillustration and discussion, and so as not to obscure the invention.

[0019]FIG. 5 is an assembled convex IHS 10 in an example embodiment ofthe present invention. It should be noted that FIGS. 5 and 6 areprovided as merely example embodiments of the present invention. Aspreviously discussed depending on the material composition of theintegrated heat spreader (IHS) 10 and the thermal interface material(TIM) 20, air gaps 60 may form anywhere in the interface between the TIM20 and IHS 10 due to separation of TIM 20 from IHS 10 caused by thermalexpansion and contraction. Further, as previously discussed air gaps mayalso form in the TIM 20 itself.

[0020] Still referring to FIG. 5, it should be noted that IHS 10 has aconvex shape in which the middle portion 70 of the IHS 10 isthicker/wider than the end portions 80 of the IHS 10. This convex shapeof the IHS 10 is done in order to increase the thickness of the TIM 20at both its respective ends 90 while allowing the middle section 100 toremain unchanged in thickness so that the air gaps seen in FIG. 3 areless likely to materialize. This is due to the fact that as thethickness of the TIM 20 increases so does its elasticity and thereforeit can better withstand the thermal stresses causing detachment as shownin FIG. 3. This is particularly true in the case where the TIM 20 ismade of a polymer metal combination or where the TIM 20 comprises asolder composite material. However, as previously discussed thisparticular embodiment of the present invention shown in FIG. 5 isspecifically designed to alleviate the detachment problems as shown inFIG. 3 and are merely provided as an example of the present invention.Those features not discussed in reference to FIG. 5 remain unchangedfrom those in FIGS. 2 and 3.

[0021]FIG. 6 is an assembled concave IHS 10 in which the center portion70 is thinner and the end portions 80 are thicker or remain unchanged insize in an example embodiment of the present invention. This concaveshape of the IHS 10 is done in order to increase the thickness of theTIM 20 in its center portion 100 while maintaining both end portions 90at near the same size so that the air gap seen in FIG. 4 is less likelyto materialize. This is due to the fact that as the thickness of the TIM20 increases so does its elasticity and therefore it can betterwithstand the thermal stresses causing detachment as shown in FIG. 4.This is particularly true in the case where the TIM 20 is made of apolymer metal combination or where the TIM 20 comprises a soldercomposite material. However, as previously discussed this particularembodiment of the present invention shown in FIG. 6 is specificallydesigned to alleviate the detachment problems as shown in FIG. 4 and aremerely provided as an example of the present invention. Those featuresnot discussed in reference to FIG. 6 remain unchanged from those inFIGS. 2 and 4.

[0022]FIG. 7 is a flowchart of a process to determine the stress pointsin a TIM 20 and modify the IHS 10 to compensate for the stress points inan example embodiment of the present invention. Processing begins inoperation 700 and immediately proceeds to operation 710. In operation710 the assembly package having a flat IHS 10 and using factorymaterials is assembled as shown in FIG. 2. In operation 720, thematerial properties and package geometry are determined. This wouldinclude determining such factors as coefficient of thermal expansion,modulus, stiffness, warpage, thickness, etc. Thereafter, in operation730 a mechanical model is created to determine the impact of packagestress on the thermal interface material (TIM) 20 during temperaturecycling. This mechanical model would comprise building a statisticallysignificant number of the packages and then cycling them through thetemperature extremes that would be experienced during a normal lifetimeof operation. Thereafter, in operation 740 the location in the TIM 20that has the greatest tensile and sheer stress applied there to isdetermined. As would be appreciated by one of ordinary skill in the artthis may be determined in a number of ways. For example, cross-sectionsof the assembled die 50, substrate 30 and IHS 10 may be examined,photographed and the location of the amount of separation determined. Inaddition, as would be appreciated by one of ordinary skill in the art,acoustic and x-ray analysis may be used to determine the location of anyseparation or any air gaps that form in the TIM 20. In operation 750,the IHS 10 is redesigned to increase the thickness of the TIM 20 in theareas of high stress as evidenced by separation points discovered inoperation 740. In operation 760 the package is assembled with theredesigned integrated heat spreader (IHS) 10 is manufactured insufficient quantity to provide statistically significant data and theretested in temperature cycling while being compared with the modeledresults. Thereafter, in operation 780 the material properties andpackage geometry of the IHS 10, TIM 20 and die 50 are determined. Thematerial properties would include, but not limited to, the coefficientof thermal expansion, modulus, stiffness, warpage, to thickness, andetc. If the material properties are within predetermined desired limits,then processing proceeds to operation 770 where processing terminates.However, if the material properties are not within the requiredtolerances, then processing loops back to operation 730 and repeatsoperations 730 through 780 until the material properties fall withinspecified limits.

[0023] The benefit resulting from the present invention is that asimple, reliable, device and method is provided for identifying andcompensating for stress points that develop between an IHS 10 and TIM 20that cause air gaps 60 to form and prevent effective heat transfer froma die 50 to an IHS 10. This device and method compensates for tensileand shear stress due to heating related expansion and contraction byplacing larger quantities of the TIM 20 at those tensile stress pointsthereby increasing the elasticity of the TIM 20 at those tensile stresspoints.

[0024] While we have shown and described only a few examples herein, itis understood that numerous changes and modifications as known to thoseskilled in the art could be made to the example embodiment of thepresent invention. Therefore, we do not wish to be limited to thedetails shown and described herein, but intend to cover all such changesand modifications as are encompassed by the scope of the appendedclaims.

We claim:
 1. A method of compensating for stress points between anintegrated heat spreader and thermal interface material, comprising:exposing the integrated heat spreader connected to the thermal interfacematerial to temperature cycling; determining at least one location inthe thermal interface material with the greatest tensile and sheerstress; and altering the shape of the integrated heat spreader toincrease the thickness of the thermal interface material at the leastone location in the thermal interface material with the greatest tensileand sheer stress.
 2. The method recited in claim 1, wherein the thermalinterface material connects a die to the integrated heat spreader andconducts heat from the die to the integrated heat spreader.
 3. Themethod recited in claim 2, wherein prior to exposing the integrated heatspreader connected to the thermal interface material to temperaturecycling, the method further comprises: creating the integrated heatspreader having an inner an outer surface which are flat, wherein theinner surface of the integrated heat spreader is connected to thethermal interface material.
 4. The method recited in claim 3, furthercomprising: determining the material properties and geometry of theintegrated heat spreader and the thermal interface material.
 5. Themethod recited in claim 4, wherein the material properties and geometryof the integrated heat spreader and thermal interface materialcomprises: determining the coefficient of thermal expansion, modulus,stiffness, warpage, and thickness of the integrated heat spreader andthe thermal interface material.
 6. The method recited in claim 5,wherein determining at least one location in the thermal interfacematerial with the greatest tensile and sheer stress, comprises: takingcross-sections, acoustic analysis, or x-ray failure analysis of theintegrated heat spreader and thermal interface material and measuringany separation between the integrated heat spreader and the thermalinterface material or air gaps formed in the thermal interface material.7. The method recited in claim 6, wherein after altering the shape ofthe integrated heat spreader to increase the thickness of the thermalinterface material, the method further comprising: adhering to theintegrated heat spreader with the altered shape to the thermal interfacematerial; and temperature cycling of the integrated heat spreaderadhered to the thermal interface material.
 8. The method recited inclaim 7, wherein after temperature cycling of the integrated heatspreader adhered to the thermal interface material, the method furthercomprising: determining the material properties and geometry of theintegrated heat spreader and the thermal interface material; anddetermining whether separation of the integrated heat spreader and thethermal interface material has occurred or air gaps have formed in thethermal interface material.
 9. A package, comprising: a die havingcircuitry generating heat; a thermal interface material connected to thedie; and an integrated heat spreader connected to the thermal interfacematerial having a shape that increases the amount of thermal interfacematerial between the integrated heat spreader and the die at a point oftensile stress between the thermal interface material and the integratedheat spreader.
 10. The package as recited in claim 9, wherein thethermal interface material conducts heat from the die to the integratedheat spreader.
 11. The package recited in claim 10 wherein the point oftensile stress is determined by physical examination of the integratedheat spreader and thermal interface material to determine a point atwhich the thermal interface material has disconnected from theintegrated heat spreader.
 12. The package recited in claim 11, whereinthe integrated heat spreader is shaped to increase the amount of thermalinterface material at the point at which the thermal interface materialhas disconnected from the integrated heat spreader.
 13. The packagerecited in claim 12, wherein the point at which the thermal interfacematerial has disconnected from the integrated heat spreader comprises aplurality of points at which the thermal interface material hasdisconnected from the integrated heat spreader.
 14. The package recitedin claim 13, wherein the amount of change the shape of the integratedheat spreader is based upon material properties and geometry includingthe coefficient of thermal expansion, modulus, stiffness, warpage, andthickness of the integrated heat spreader and the thermal interfacematerial.
 15. A method of compensating for stress points between anintegrated heat spreader and thermal interface material, comprising:creating the integrated heat spreader with a flat interior surface;connecting the interior surface of the integrated heat spreader to thethermal interface material; exposing the integrated heat spreaderconnected to the thermal interface material to temperature cycling;determining at least one location in the thermal interface material withthe greatest tensile and sheer stress; and altering the shape of theintegrated heat spreader to increase the thickness of the thermalinterface material at the least one location in the thermal interfacematerial with the greatest tensile and sheer stress.
 16. The methodrecited in claim 15, wherein the thermal interface material connects adie to the integrated heat spreader and conducts heat from the die tothe integrated heat spreader.
 17. The method recited in claim 16,further comprising: determining the material properties and geometry ofthe integrated heat spreader and the thermal interface material.
 18. Themethod recited in claim 17, wherein the material properties and geometryof the integrated heat spreader and thermal interface materialcomprises: determining the coefficient of thermal expansion, modulus,stiffness, warpage, and thickness of the integrated heat spreader andthe thermal interface material.
 19. The method recited in claim 18,wherein determining at least one location in the thermal interfacematerial with the greatest tensile and sheer stress, comprises: takingcross-sections, acoustic analysis, or x-ray failure analysis of theintegrated heat spreader and thermal interface material and measuringany separation between the integrated heat spreader and the thermalinterface material or air gaps formed in the thermal interface material.20. The method recited in claim 19, wherein after altering the shape ofthe integrated heat spreader to increase the thickness of the thermalinterface material, the method further comprising: adhering to theintegrated heat spreader with the altered shape to the thermal interfacematerial; and temperature cycling of the integrated heat spreaderadhered to the thermal interface material.
 21. The method recited inclaim 20, wherein after temperature cycling of the integrated heatspreader adhered to the thermal interface material, and method furthercomprising: determining the material properties and geometry of theintegrated heat spreader and the thermal interface material; anddetermining whether separation of the integrated heat spreader and thethermal interface material has occurred.